1. Field of the Invention
The present invention relates to grinding apparatus, and particularly to an apparatus for grinding and polishing facets in precious stones and the like so as to repeatably maintain constant facet angles during each grinding or polishing operation.
2. Description of the Prior Art
Theretofore it has been well known in the art to have an apparatus providing a substantially horizontally planar grinding or polishing surface having a substantially circular area, which surface is driven to rotate in a substantially horizontal plane about the centroid of the area. A gem stone or other item to be ground or polished so as to have a finished facet is held in a fixed orientation against the grinding or polishing surface. Several means have been devised to accomplish the latter feature, as are briefly described in the patents discussed below.
U.S. Pat. No. 1,461,149, issued on July 10, 1923 to A. T. Hunt, discloses an Apparatus for Cutting Diamonds. Without distinguishing between those features described in this patent as "old" and those features alleged to be "new", the apparatus can be considered as a combination of two subsystems, one driving the cutting wheel and the other positioning the diamond to be cut. The wheel driving system has a cutting wheel, having a substantially planar horizontal cutting surface, mounted on a vertical axis, about which axis the wheel is rotated by an electric motor. The wheel, its axis, and the electric motor are, in turn, rotated in a planetary manner about a vertical axis proximate to, but within, the periphery of the grinding wheel. The diamond to be cut is held against the cutting surface of the cutting wheel at substantially the centroid of the planetary rotation by a pivotable supporting structure which allows the diamond to be lifted from contact with the cutting surface, examined, and returned to the cutting surface for further cutting on the same facet. The support means also provides for adjustment in the angular position at which the stone is held and also in the orientation of the stone about an axis through the support structure.
The latter feature can be better understood through a brief general discussion of the nature of cutting facets on gem stones. In substantially all of the several forms of apparatus for cutting such facets that have been found in the prior art, the stone to be cut is rigidly held on or within a chuck or a "dop". This chuck or dop is then supported over the cutting wheel at an angle appropriate to the desired facet to be cut. Changing the angle of support provides for the cutting of a differing facet angle. Rotation of the chuck or dop about an axis of support thereof through an appropriate angle of rotation enables a plurality of facets having the same facet angle to be cut on the surface of the gem stone. In the Hunt apparatus, a further adjustment must be made in the horizontal positioning of the support structure as the facet angle is changed so that portion of the surface of the stone to be cut is brought into substantial vertical registration with the centroid of the planetary rotation of the cutting wheel system.
U.S. Pat. No. 3,811,230, issued on May 21, 1974 to A. Beck for a Facet Grinding Apparatus, discloses the use of a parallelogram lever arrangement to establish and maintain a particular facet angle at which the work is held against the cutting wheel to enable cutting a plurality of facets of equal facet angle around the periphery of the gem stone being cut. Several indexing means are provided to indicate the facet angle, the amount of gem stone material removed during cutting, and the rotational position of the facet about the axis of support of the dop holding the gem stone.
One approach to cyclical control of gem grinding is disclosed by the Gem Grinder With Approach Control Means of P. D. Bean in U.S. Pat. No. 3,902,283. issued Sept. 2, 1975. In this system, a dop holder is supported by a mechanized, variably positionable holder cantilevered from an arcuate structural arm extending upwardly to a distal end over a cutting wheel surface. The arcuate arm is coupled to a vertical mechanism which provides for accurate adjustment and control of the vertical extent of the arcuate arm. The vertical support mechanism is also provided with horizontal motion drive and control to appropriately position the arcuate arm over the surface of the cutting wheel. Once a gem stone has been appropriately mounted on a dop and placed in the dop holder riding on the arcuate arm, the controls are set for a desired facet angle and rotational orientation of the facet. Activation of the system then causes the vertical support to move upwardly, and the dop holder mechanism to ride up the arcuate arm so that the appropriate facet angle is attained between the axis of the dop holder and the surface of the cutting wheel. The vertical support is then caused to translate horizontally to bring the stone over the cutting wheel, at which time the vertical support is again lowered to bring the stone into contact with the cutting surface. A pressure sensitive driving mechanism is contained within the dop holder to axially drive the stone against the cutting wheel surface, maintaining a appropriate cutting pressure throughout. Strain guages and limit switches are employed to appropriately limit the cutting. When a first facet has been cut at a given angle, the dop holder is returned to its initial axial position, the vertical support is upwardly extended, and the dop holder is caused to rotate about its axis to attain the orientation of the subsequent facet, whereat the vertical arm is again lowered and the dop holder is axially extended through completion of the cutting of this facet. Cutting of facets at a second and subsequent facet angles is accomplished and controlled in a like manner except that for each change in facet angle, the vertical and horizontal positioning of the vertical support of the arcuate arm is appropriately adjusted.
Other gem stone cutting devices are known, but the aforementioned examples represent the extent of the prior art most closely approaching the present invention of which the applicant is presently aware. Such other systems and devices range from a simple cutting wheel against which a gem stone is substantially manually held, to apparatus wherein a dop holding the stone being cut is manually repositioned and clamped for each facet to be cut. None of such other known apparatus provides for the accuracy and repeatability of the specific examples described hereinabove.